What assistance is available, if any?

I've found that when going to somewhere crowded, like a concert venue, my children sometimes find it easier if we get there early, before the crowds build. They enter a venue that isn't busy and are more able to adjust as it gradually fills.

Others have commented about leaving early but if she's not willing to do that you can always try to leave late. Venues tend to empty very quickly so you won't be hanging around for ages. Anything that avoids being part of the mass of people can help.

If she's getting overloaded then consider going to a first aid room. It might be busy but there's a reasonable chance that it will offer a space without too much stimulation.

I have ASD and so does my daughter. My advice is this:

Plan everything in advance and talk about it loads together, look at pictures of the venue and of crowds, listen to the music whilst looking. Plan everything you are doing, how to get there, where you are going to eat, sleep etc. That way there is less that is uncertain and unpredictable for her. The more she can imagine it in advance, the easier I think it can be. In terms of being in London, plan for managing the stress of all the people too, consider ear plugs, sunglasses, wearing a baseball cap (it limits your vision and you can control how much you can see in front of you), maybe have some chillout music with you to listen to on headphones. When she has a meltdown, just accept it, it's how she resets herself in stressful situations and comes to terms with things. She survives them fine and so do you so don't make a big deal. Autism is really well known about in the UK and if you need help in train stations and at venues, people are usually happy to assist. Give her time to get used to things and to get in to things, she may just need space to find her own way with things. Hope it goes well, I took my daughter to London recently and after a meltdown on arrival it went really well and she loved it. She just needed to let her feelings out and get used to a new environment. Change is hard for us Auties, and even when we want the change (in environment for example - like going to London for a concert), that doesn't make our brain any different, our brains need time to adjust but definitely planning and preparing helps loads. 

Have a look at herbal remedies for anxiety and stress. For instance, ginger works as a mild sedative and can aid sleep, I’ve been using it instead of sleeping tablets to help my insomnia and I’ve had amazing results! Homeopathic chemists or herbalist would offer a wide range of natural remedies that may help. I hope you both have a wonderful time at the concert 

Most venues have disabled areas - speak to the venue and explain the situation - they will probably seat you in that area.    They are likely to treat you the same as UK fans - they will likely only charge for her ticket as her carer will get in for free.

Be prepared to leave early if it's too much for her and also plan you travel with great detail - with escape routes if she gets overwhelmed.

I recommend leaving before the last song to get ahead of the crowds if you want to avoid a long journey and tight crowds.

If you will have a car, same rule applies - get out early to avoid getting stuck in traffic.  Better to enjoy 90% that have it ruined by a meltdown in the crowds.

Spend time explaining to her what is going to happen on the evening - get on the internet and use google maps to explore the area so there are less surprises on the night.   Make sure she understands that if she's had enough, you will get her to safety before it becomes a big problem for her.

Be prepared to bail out before getting to the venue if she overloads.

Hope it all goes well for you!